1. Field of the Invention
The present invention relates to an image forming apparatus including a resist rotary member that feeds out a recording member to a belt member and a fixing rotary member that fixes a toner on the recording member.
2. Description of the Related Art
An increase in printing speed of color image forming apparatuses is strongly demanded. To meet this demand, so-called tandem type color image forming apparatuses have become popular. These apparatuses employ a direct transfer system or an intermediate transfer system in which transfer areas for a plurality of image carriers are located on a feed path for a recording member. The tandem type color image forming apparatus using the direct transfer system feeds a recording member by carrying it on a surface of a paper feeding belt (recording member feeding unit). A color image is formed on the recording member by sequentially transferring toner images on individual image carriers onto the recording member, which is fed out by a resist roller (resist rotary member) and conveyed on the paper feeding belt, one on another. The tandem type color image forming apparatus using the intermediate transfer system sequentially transfers toner images on individual image carriers onto an intermediate transfer body one on top of another. The color images on the intermediate transfer body are transferred at one time onto a transfer member fed out by the resist roller.
In the tandem type color image forming apparatus, if the velocity of the circumferential surface of the resist roller (resist linear velocity) differs from the surface velocity of the paper feeding belt (belt moving velocity), the color registration can be shift.
The following describes why the color registration shifts in the tandem type color image forming apparatus employing the direct transfer system having four image carriers when the resist linear velocity differs from the velocity of the paper feeding belt. In the following explanation, a first image carrier, a second image carrier, a third image carrier, and a fourth image carrier are laid out from the resist roller in this order.
An example in which the resist linear velocity is set faster than the belt moving velocity of the paper feeding belt is explained first. A recording member fed out from the resist roller adsorbs on the paper feeding belt, and is fed to a transfer area of each image carrier according to the surface movement of the paper feeding belt. Ideally, the recording member and the paper feeding belt are completely in contact with each other and are not influenced at all by disturbance, in which case, color registration is hardly shifted. However, in reality, disturbance causes sliding of several micrometers to several hundred micrometers between the recording member and the paper feeding belt. The disturbance may change the load applied to the paper feeding belt, so that the belt moving velocity changes. The disturbance that causes such sliding or a change in the belt moving velocity is mainly the influence of a resist roller that is driven at a resist linear velocity that does not coincide with the belt moving velocity of the paper feeding belt. Specifically, when a recording member is fed out from the resist roller which is driven at a resist linear velocity Vr, the recording member adsorbs on the paper feeding belt driven at a belt moving velocity Vt (Vt<Vr). The moving velocity of the part of the recording member which adsorbs on the paper feeding belt is Vta (Vt<Vta<Vr), not Vt that is the same as the belt moving velocity, and the leading end of the recording member enters the transfer area of the first image carrier at the moving velocity Vta. As the recording member is fed thereafter, the contact area between the recording member and the paper feeding belt increases, so that the moving velocity of the recording member is dominated by the paper feeding belt rather than by the resist roller. By the time the leading end of the recording member reaches the transfer area of the fourth image carrier, the moving velocity of the recording member approximately matches with the belt moving velocity Vt of the paper feeding belt. In the tandem type color image forming apparatus using the direct transfer system, if the moving velocities of the recording member when passing the transfer areas of the individual image carriers do not coincide with one another, the color registration shifts. In the above example, the moving velocity of the recording member is Vta when passing the transfer area of the first image carrier, becomes slower gradually thereafter, and becomes Vt when passing the transfer area of the fourth image carrier. Accordingly, toner images of the individual colors to be transferred from the respective image carriers are transferred at positions shifted from one another by that difference, resulting in the shift of color registration.
An example in which the resist linear velocity is set slower than the belt moving velocity of the paper feeding belt is explained. When a recording member is fed out from the resist roller which is driven at a resist linear velocity Vr, the recording member adsorbs on the paper feeding belt driven at a belt moving velocity Vt (Vt>Vr). The moving velocity of the part of the recording member which adsorbs on the paper feeding belt is Vta′ (Vt>Vta′>Vr), not Vt that is the same as the belt moving velocity, and the leading end of the recording member enters the transfer area of the first image carrier at the moving velocity Vta′. As the recording member is fed thereafter, the contact area between the recording member and the paper feeding belt increases, so that the moving velocity of the recording member is dominated by the paper feeding belt rather than by the resist roller. By the time the leading end of the recording member reaches the transfer area of the fourth image carrier, the moving velocity of the recording member approximately matches with the belt moving velocity Vt of the paper feeding belt. The moving velocity of the recording member is Vta′ when passing the transfer area of the first image carrier, becomes faster gradually thereafter, and becomes Vt when passing the transfer area of the fourth image carrier. Accordingly, toner images of the individual colors to be transferred from the respective image carriers are transferred at positions shifted from one another by that difference, resulting in the shift of color registration.
The shift of color registration can be also caused by disturbance originating from the linear velocity of a fixing rotary member, such as fixing rollers that hold the recording member at the downstream of the paper feeding belt in the feed direction of the recording member.
The following technique is conventionally known to prevent such out of color registration. For example, the recording member feeding velocity of the resist roller is set slightly faster than the recording member feeding velocity of the paper feeding belt (transfer belt), and the resist roller is disposed askew in the vertical direction with respect to the recording member inlet port of the paper feeding belt. The velocity setting and the layout of the resist roller flex the recording member between the paper feeding belt and the resist roller to absorb the difference between the belt moving velocity and the velocity of feeding the recording member by the resist roller. This technique is also adaptable to the velocity difference between the fixing rotary member and the transfer belt.
To downsize an image forming apparatus, however, the distance between the paper feeding belt and the resist roller, and the distance between the paper feeding belt and the fixing rotary member should be made shorter. This makes it difficult to secure a sufficient space for flexing the recording member to absorb the velocity difference between them. When the recording member has a high rigidity to flexibility in the feed direction, such as thick paper, even when the recording member is flexed between the paper feeding belt and the resist roller, the rigidity causes the disturbance to be transmitted to the paper feeding belt. The diameter of the resist roller can change due to the environment, such as temperature and humidity, and a frictional force between the recording member and the resist roller can change due to aging abrasion, or adhesion of paper dust or the like, which can change the feeding velocity of the recording member by the resist roller. It is therefore difficult to keep the initially set moving velocity of the paper feeding belt and the initially set recording member feeding velocity of the resist roller for a long period.
As a solution to the problem originating from the feeding of the recording member in a flexed manner, Japanese Patent Application Laid-Open No. 2004-151382 discloses a scheme using a detector that detects a rotational velocity of a transfer belt (paper feeding belt), and a detector that detects a feeding velocity of a recording paper. The scheme detects the moving velocity of the transfer belt, and the recording member feeding velocity of the resist roller, and controls the rotational velocity of the resist roller so that the recording member feeding velocity of the resist roller becomes slightly faster than the moving velocity of the transfer belt with a predetermined velocity difference maintained.
However, comparison of the velocities detected by the two detectors proposed in Japanese Patent Application Laid-Open No. 2004-151382 requires high precision of both detectors. For example, there is a method of measuring the feeding velocity of a recording member fed out by the resist roller. The method uses two optical sensors, laid out in parallel in the feed direction to measure the time for the leading end or the trailing end of the recording member to pass the two sensors. A calculation of the feeding velocity of the recording member from the measured pass time requires an exact distance between the two sensors. The moving velocity of the transfer belt can be measured by a method of measuring the amount of the rotation of an adsorption roller that rotates with the surface movement of the transfer belt. The adsorption roller is provided to face a driven roller with the transfer belt provided in between, and electrostatically adsorbs the recording member to the transfer belt. A calculation of the moving velocity of the transfer belt from the rotational amount of the adsorption roller requires an exact value of the circumferential length of the adsorption roller. When the exact values are not known in advance, the set value of the rotational velocity of the resist motor does not become a proper value. The distance between the two sensors differs from one product to another due to a difference in mounting precision. The circumferential length of the resist roller differs from one product to another due to a difference in the precision of parts. It is therefore difficult to calculate the adequate set values for each individual product.